The present invention relates to a new and improved method of synchronizing a deciphering device operating as a receiver with an enciphering device operating as a transmitter, and furthermore, the present invention relates to apparatus for the performance of the aforesaid method.
Enciphering and deciphering devices which cooperate in a digital network as transmitter and receiver, respectively, are conventionally equipped with Key pulse Generators which, as a general rule are stepped forward for each character pulse to be enciphered and deciphered respectively. Consequently, synchronism of the receiver end Key Generator, required for proper deciphering, with the Key Generator at the transmitter end is ensured. Of course, there is presupposed that at the start of the transmission of the message the Key Generators at the transmitter end and receiver end have the same starting position.
As an example there will be discussed hereinafter a presently employed technique for synchronizing the Key Generators at the receiver station and transmitter station. As a general rule the Key Generators are programmed with a secret base information, the Basic Key, which is valid for a certain period of time for all devices hooked up to the network. Starting from this base information there is determined for each transmitted message a certain starting information, commonly called the Message Key. The starting information need not of necessity be secret, provided that knowledge thereof without the base information is useless to an unauthorized individual. In practice such Message Key information is placed ahead of the actual message. By means of this MK receiver end device is brought into the same starting position as the transmitter end device.
The synchronization of the Key Generators at the transmitter end and receiver end at the start of the transmission of a message or text is especially problematic in the case of meshed or interconnected networks where messages of high information content or density are transmitted in various directions always between different stations. If, for instance, station A has transmitted a message during a predetermined time duration T to station B, then, as explained above, the Key Generators of these stations A and B are advanced through a number of steps corresponding to the length of the transmitted message. Now if station A or station B is supposed to transmit a message to station C which was not operational during the aforementioned time duration T, then, first of all the Key Generator of such station C must be synchronized with the Key Generator of station A or station B, as the case may be. This synchronization or placing in step of the Key Generators with one another must occur very rapidly.